1. Field of the Invention
The present invention relates to an IR gas analyser window for particular use with an adapter of an IR gas analyser for the analysis of respiratory gases. The invention also relates to a particular method of producing such a window.
2. Description of the Related Art
Main flow measuring gas analysers calculate the composition of the respiratory gases of a patient, by sending infrared light through the respiratory tract of the patient and therewith measuring the absorption of said light at chosen wavelengths. For the sake of accuracy, main flow measuring gas analysers are placed as close as possible to the patient's mouth.
In order to enhance the efficiency of such respiratory gas measuring processes, there has recently been adopted the use of adapters that are inserted in the respiratory circuit and through which the respiratory gases are allowed to pass, while, at the same time, coupling a measuring instrument to the adapter for registering the gases that pass therethrough.
A gas analyser adapter is known from, for instance, the International Patent Application PCT/SE02/01528 (Publication No. WO 03/017837). This adapter is intended for co-action with a measuring head known from the International Patent Application PCT/SE02/01946 (Publication No. WO 03/060490). The adapter includes a window which constitutes a delimitation of a gas passageway through which the gas to be analysed flows, and the analysis is effected by sending a beam of infrared light through a window on one side of the gas passageway and further through the gas present in said passageway, and through a corresponding window on the other side of the gas passageway, where said beam is received by an IR detector.
Similar adapters are known, for instance, from U.S. Pat. Nos. 5,616,923; 6,095,986; 6,216,692; and 6,258,040.
A particular problem that has been observed in respect of the aforesaid types of adapter resides in the moisture deposits that can occur on the inside of the adapter walls. These deposits are due to the fact that the respiratory gases contain a great deal of moisture and because the moisture contained condenses inside the adapter due to the difference in temperature between the respiratory gas present in the adapter and the considerably colder ambient air on the outside of the adapter, wherein said moisture is deposited in water droplet form, inter alia on the windows in the adapter.
The windows through which the IR rays shall pass have been found to be a problematic source in those adapters of the aforesaid kind known hitherto. The requirements that should preferably be placed generally on the windows included in adapters of the aforesaid kind are:                That the windows shall allow IR rays to pass through in the desired wavelength range.        That the windows shall positively eliminate any condensation problems emanating from the gases in the respiratory circuit.        That the windows shall not be deformed by mechanical action or as a result of pressure changes in the respiratory circuit.        That surface collection of liquid shall be counteracted.        That the windows can be mounted in the adapter in a gas-tight manner.        That the windows have a low production cost.        That it shall be possible to mount the windows in the adapter in a simple and inexpensive manner.        
The following problems have been observed among the various solutions proposed with regard to window designs:                Traditional gas windows are expensive and often need to be heated in order to avoid moisture deposits. Heating has the additional disadvantage of utilising power and also lengthens the time before which the sensor can begin to be used. Heating also results in different material compositions that make recovery of the adapters used difficult to achieve.        Foils that are sufficiently strong mechanically normally have a poor transmission capacity in high wavelength regions. It has also been found difficult to flatten foil, therewith requiring some form of stretching or tensioning means, which, in turn, renders the elements twist responsive. Normally, additional fastener elements are required to mount the foils flush with the inner wall, so as to prevent the formation of pockets that are able to collect liquid around the window. It is often also necessary to glue these additional fastener elements, in order to obtain a secure and tight fastening.        The different component parts and the mounting steps involved make manufacture of the product expensive. As mentioned above, the use of different materials also results in recovery problems.        